I am well aware of how the oil system works in an internal combustion engine. Oil doesn't get 'lost', it gets circulated because the system is designed to continuously pump the oil to through the system, kind of like the cooling system.
Actually, the oil does get "lost", or forced from the bearing surfaces (preferably by oil pressure), where it drains back into the pan, to be recirculated by the pump again. This is different from a sealed system, in which the lube stays in the bearing (but you already stated that below......)
Theoretically (that term has been used already) the engine could stay lubed with a closed pressurized system if the oil could stay cool and clean enough. That is too expensive to implement and hurts the oil dudes too much also.
The oiling system on a car is rudimentary. Maintain certain clearances and keep pumping oil through it
It is wildly impracticable to attempt to run a sealed pressurized system in a standard auto, and a non-pressurized (sealed bearing) system will not work for babbitt style bearings.
Why would you want that high of a pressure???? HV pump is overkill especially on a 2.5. All that does is take hp away from the drivetrain and puts stress on the cam. Its a psych thing.
Again, do not confuse a high volume
pump with a high pressure
Higher pressure in a pump is achieved with a heavier bypass spring. This can be achieved with both a stock or high volume pump. Generally, there is little reason to run higher pressure on a standard engine, as all you're doing is putting a strain on your cam and distributor drive gears.
Higher volume is achieved with larger gears. A high volume pump will push more oil at the same pressure when compared to a stock pump. This is important when running engines that may see very low RPM's. While a stock pump may be running a constant 40LBS at idle (550 or so), it may drop drastically at 250-350RPM's. This may be a problem with engines that encounter moments of very low RPM's lugging over a technical obstacle. A high volume pump will be able to maintain volume (pressure) at these low speeds due to the added amount of oil it is trying to pack into the journals. Many HV pumps come with one or more pressure springs (my Melling came with three) offering a variety pressures.
On a worn engine, many times a high volume pump will compensate for excessive clearances by pushing more oil into the voids. A high pressure pump will not be effective at this until the gears rotate fast enough to create the volume necessary to overcome bleedoff (loss).
Contrary to what is being said, neither a high volume or a high pressure pump needs a larger pan. Neither of these pumps can move more oil than the engine clearances will allow, with the excess oil being route back into the pan via the bypass valve.